Added two more time spans to power.ui

I felt the personal need for 15 minutes and 45 minutes time spans in the power module, so I added it. (E.g. I want my screen switched off after 15 minutes)
Actually I'd prefer a GtkSpinButton, which leaves the user more choice, but I'm fairly new to GTK and I didn't understand how the SpinButton exactly works. So I left it with the additional time spans.

Modifications tested on Linux Mint 17 Cinnamon Edition RC